1. Field of the Invention
The present invention relates to a positive electrode, for use in a lithium secondary battery, having a large capacity and enhanced reliability in a nail penetration test, and to the lithium secondary battery using the positive electrode.
2. Description of the Related Art
Lithium secondary batteries are used as power sources for driving laptop personal computers, portable communication devices, and the like. In recent years, development of portable or wireless electronic devices has increased a demand for large-capacity, compact, and lightweight electronic devices. In response to the demand, large-capacity lithium secondary batteries have been developed thanks to the improvement or modification of electrode materials, improvement of battery construction, and the like. The expansion of a battery capacity leads to an increase of an energy density of the battery, which increases a demand for enhanced reliability of the battery upon releasing a large energy in an internal short-circuit test or the like. In view of this, there is a strong demand for lithium secondary batteries that meet the requirements for high reliability in the internal short-circuit test concurrently with those for large capacity.
There is known an internal short-circuit test to penetrate a nail into the battery (hereinafter, called as “nail penetration test”), as one of the tests to check reliability of lithium secondary batteries at the internal short-circuiting. Once a positive electrode initiates its thermal decomposition by the internal short-circuiting in a nail penetration test, the lithium secondary battery with a large energy density releases a large energy, which may overheat the battery by thermal runaway. The overheating of the lithium secondary battery in the nail penetration test is greatly influenced by thermostability of the positive electrode.
Thermostability of the positive electrode depends on thermostability of an active material used for the positive electrode. Known examples of the active materials used for the positive electrode in the lithium secondary battery are lithium-containing composite oxides such as LiCoO2 or LiNiO2 having a layered structure, and LiMn2O4 having a spinel structure. These lithium-containing composite oxides are different from each other in electrochemical properties and thermostabilities.
For instance, a lithium nickel oxide such as LiNiO2 has a reversible capacity of 180 to 200 mAh/g, and has a larger capacity density, a lower exothermic initiation temperature and a lower thermostability than the other lithium-containing oxides. Therefore, lithium secondary batteries using the lithium nickel oxide as the positive electrode active material tend to be overheated in a nail penetration test.
In view of the above, there are proposed lithium secondary batteries using, as the positive electrode active material, a lithium-containing composite oxide obtained by partially substituting the nickel component of LiNiO2 with other element, and lithium secondary batteries using, as the positive electrode active material, a mixture of a lithium nickel oxide and a lithium-containing composite oxide having a higher thermostability than the lithium nickel oxide to suppress oxygen release at the thermal decomposition so as to improve thermostability of the positive electrode while maintaining the large capacity of the positive electrode.
For instance, Japanese Unexamined Patent Publication No. 2003-36838 discloses a lithium secondary battery with a positive electrode constructed as follows. The positive electrode has two or more mixture layers, on a surface of a positive electrode current collector, containing a lithium-containing composite oxide as a positive electrode active material, wherein a positive electrode active material having a higher exothermic initiation temperature is used for the outermost mixture layer. According to the conventional art, in conducting a nail penetration test, instantaneously when a nail, which assumes a negative electrode potential by penetrating through the negative electrode, contacts the positive electrode, a large current flows through the outermost surface of the positive electrode, which generates Joule heat. The above construction has been proposed to prevent thermal decomposition of the positive electrode by the Joule heat.
The nail penetration test is conducted to check whether a battery has overheated by intentionally generating an internal short-circuit, which may occur by intrusion of foreign matters or the like. In view of this, it is desirable to conduct a nail penetration test under an internal short-circuit condition corresponding to an assumed most severe use environment. For instance, in the case where a nail penetration speed is slow in the nail penetration test, an internal short-circuiting occurs with higher probability than a case where the nail penetration speed is fast. As a result, a current is collected at a short-circuited portion, which may overheat the battery. A lithium secondary battery with a large capacity has a strong demand for suppressing overheating in the nail penetration test under the above-mentioned severe internal short-circuit condition.
According to the detailed research and development on the lithium secondary batteries by the inventors, however, it could not be confirmed that overheating of the lithium secondary battery disclosed in the above publication was remarkably suppressed in the nail penetration test under the aforementioned severe internal short-circuit condition.